Fig. 1032.12 Convex and Concave Sphere Packing Voids:
 In a tetrahedron composed of four spheres, the central
void is an octahedron with four concave
spherical triangular faces and four planar triangular
faces with concave sides. This can be described
as a "concave octahedron."
 In an octahedron composed of six closepacked spheres,
the central void is a vector equilibrium with
six concave spherical square faces and eight triangular
faces with concave sides: a "concave vector
equilibrium."
 The vector equilibrium with edges arced to form a
sphere: a convex vector equilibrium.
 The vector equilibrium with arcs on the triangular
faces defined by spheres tangent at vertexes: a
concave vector equilibrium.
